Drive Channels: why two gain knobs?

jaymenon

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

jasonbone75

Two stages can be voiced differently with simple component selections to build a gain structure with different frequency response so that is definitely something that can make a difference between the two approaches. 

jasonbone75

And the diehard axiom of course :-) More Gain = More Gooder lol!

darthed1981

Basically the same as having a gain on a pedal and on the amp, options-wise.

You can even do that on digital modelling amps, on the Boss Katana for example you can put a drive pedal on and still have amp gain.

jaymenon

But these are (subtractive) potentiometers - not amplification stages.

please correct me if I’m wrong

darthed1981

jaymenon said:

But these are (subtractive) potentiometers - not amplification stages.

please correct me if I’m wrong

You want my learned friend @ICBM ;

LastMantra

I'd imagine they effect different parts of the circuit and just give you the option rather than one or the other?

ecc83

Each "gain" control sets the level that dives the next stage. One common setup is a gain pot after the first triode stage which sets the level into quite often the next stage that dives the tone stack. There then can be a pot post stack. Distortion will be different whether you distort pre or post tone stack.

I put gain in " " because the pots are actually just volume controls. Proper gain control is effected by a negative feedback circuit. Often one or both pots will have a 'situp' resistor in its bottom end to prevent total loss of signal.

Dave.

StefB

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature. 

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots. 

jaymenon

StefB said:

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature. 

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots. 

Redplate BlackVerb





Ceriatone / Dumble ODS

jaymenon

Though I wouldn't know how to adjust them to taste...

One knob seems so much simpler (despite the trouble mine has gotten me into over the course of my life)

Sassafras

LastMantra said:

I'd imagine they effect different parts of the circuit and just give you the option rather than one or the other?

Apparently, they "feel" different!

Neilybob

jaymenon said:

StefB said:

jaymenon said:

Many amps have two gain knobs on the drive channel:

Gain Control: sets the amount of signal for the first gain stage of the OD section

Drive Control: controls the level between the 2 gain stages of the OD section

In practical terms, does it make a difference if you have just one gain control?

What amps are you thinking of? I can’t call many to mind that have this feature. 

In fact the only one off the top of my head is the drive channel from Fender’s SuperSonic range, which is utter crap in no small part thanks to having two gain pots. 

Redplate BlackVerb





Ceriatone / Dumble ODS

That's interesting. My Ceriatone amp doesn't have a OD Trim, instead it has this on the back.  

jasonbone75

Ch3 on boogie mark IV (V too probably but have not looked at one)

randella

From memory, the JCM900 has gain and “sensitivity”, one controlling a valve stage and the other a secondary diode clipping stage. 

Probably a gross over-simplification

Modulus_Amps

It gives the user much more control over the amount of drive available.

The trimmer control could just be two resistors inside the amp pre set by the amp builder. 

The trimmer control is probably a set and forget type control, but if you have ever bought an amp and the drive channel has way to much gain or not enough, you would wish you could change the internal gain  - with this set up you can.

dazzajl

Surely you’d have two knobs so you can go up to 22? 

jaymenon

One knob has landed me in enough trouble…

exocet

The Dumble Overdrive Special type amp has 4 "gain stages".
The standard input volume / gain sets the amount of attenuation between stage 1 and 2 (Valve 1).
Then there is a trimmer or rear volume pot to control attenuation between stage 2 and 3 (Valve 1 to Valve 2 )
Then there's the Overdrive control (front panel) that attenuates between stage 3 and 4 which is followed by the (Ratio / level control) that acts as a Master Volume for the Overdrive signal only. It's simply a volume pot at the output of stage 4.
There's a lot of signal gain available when you cascade 4 ECC83 triodes together. The various controls allow more control at various stages in the signal chain preventing too much clipping from taking place in one part of the chain.

Ddigger

exocet said:

There's a lot of signal gain available when you cascade 4 ECC83 triodes together.

TBH, I don't know much about amps.

When you say 4 triodes, does that mean two ECC,83 valves and you are using both sides of each valve?

exocet

Ddigger said:

exocet said:

There's a lot of signal gain available when you cascade 4 ECC83 triodes together.

TBH, I don't know much about amps.

When you say 4 triodes, does that mean two ECC,83 valves and you are using both sides of each valve?

Yes. A single ECC83 / 12AX7 contains two triodes (2 gain stages).